The invention relates to a permanent magnet for connecting with a connection device of an external rotor machine, comprising                a north pole and a south pole as the magnetic poles,        wherein in a cross-section of the permanent magnet a magnetization runs from the south pole to the north pole.        
The invention also relates to a rotor for an external rotor machine comprising at least one such permanent magnet, and an external rotor machine comprising the rotor, a vehicle wheel comprising the rotor, and a wind turbine comprising the rotor.
Permanent magnets of this kind are known from EP 1276212 A2. The permanent magnets are described in connection with synchronous electric machines, in particular also external rotor motors or external rotor machines, with these being excited by permanent magnets. The efficiency of these machines is higher than that of electrically excited synchronous machines. Permanent magnets with high energy density, in other words, a high product of flux density and field strength, prove to be superior to less high-energy magnets. It is known that permanent magnets can be used not only in the form of direct assignment to the groove gap, in other words, in a flat arrangement, but also in the manner of a collector configuration (flux concentration). Direct assignment to the groove gap means that the flux density of the magnet is approximately equal to that of the groove gap. This applies at least as long as the groove gap is small in relation to the magnet height. The flux concentration arrangement allows greater flux densities in the groove gap than in the magnet. This is achieved by a large magnet arrangement. The cross-sectional area of the magnet is greater than the pole surface in the groove gap. Accordingly, the flux density in the magnet is less than that in the pole region. For increased torque utilization in the case of a synchronous machine with respect to comparable synchronous machines, optionally in addition to the specified condition for the division ratio of the mean coil width and the pole division width, EP 1276212 A2 proposes arranging the permanent magnets of the rotor in flux concentration. The groove gap induction is thereby raised to well over one tesla without the stator iron being saturated. Air gap inductions are thereby attainable, which correspond to about twice that of known electric synchronous machines. An increase in the thermal torque thereby occurs in EP 1276212 A2 with respect to conventional synchronous machines, with “thermal torque” being taken to mean the maximum continuous torque to be delivered at a predefined temperature. An increase in efficiency is therefore brought about as well as an increase in maximum torque. The utilization of the machine (Nm/kg) is increased by about a factor of 2 compared with conventional synchronous machines. The low mass also establishes an increase in the acceleration capacity.